1. Field of the Invention
The present invention relates to a method for processing a handoff in a mobile communication system, and more particularly to a method for processing a handoff in a mobile communication terminal.
2. Description of the Related Art
Conventionally, the term “handoff” means a function for continuing a call by automatically coupling a mobile communication terminal from a traffic channel of a radio cell to another traffic channel of another radio cell when a subscriber of the mobile communication terminal moves from the radio cell to another radio cell.
In a mobile communication system based on a CDMA (Code Division Multiple Access), the handoff is divided into a hard handoff, a soft handoff, and a softer handoff. The hard handoff is performed when the mobile communication terminal moves between base stations using different frequencies. In this case, an instantaneous call cutoff can occur in a call connection state, but the call can be coupled to another base station next to a corresponding base station so quickly that a caller does not experience any inconvenience during communicating with a called party. The soft handoff is performed when the mobile communication terminal moves between base stations using a same frequency. Because the mobile communication terminal moving between the base stations uses the same frequency in the soft handoff, it does not affect the call. The softer handoff is performed when the mobile communication terminal moves between sectors located within one radio cell being covered by a base station. The procedure of processing the softer handoff is similar to the procedure of processing the soft handoff.
The handoffs described above are performed between the base stations and the mobile communication terminal. A procedure of transmitting and receiving messages in a conventional handoff will be described with reference to FIG. 1. FIG. 1 is a flow chart illustrating an operation of the mobile communication terminal according to a conventional handoff process. First, the mobile communication terminal transmits a pilot strength measurement message to a base station to which the mobile communication terminal belongs in step 101. In response to the pilot strength measurement message, the base station transmits a handoff direction message to the mobile communication terminal. The mobile communication terminal receives the handoff direction message from the base station in step 103. In response to the handoff direction message, the mobile communication terminal always transmits an acknowledgement message to the base station in step 105. The mobile communication terminal is coupled to a new base station or its traffic channel to perform the handoff in step 107. The mobile communication terminal transmits a handoff completion message to the base station in step 109 and then the handoff ends.
As an improvement of an initial handoff, the conventional handoff as the FIG. 1 has improved the initial handoff by carrying the acknowledgement message with the handoff completion message after the mobile communication terminal receives the handoff direction message.
The above-described, conventional handoff may have some degree of merit in view of power saving when used in a soft handoff, but it has a drawback of having frequent call drops when used in a hard handoff, because after Tx is disabled to make a frequency change and then enabled to a new frequency, in case of the hard handoff, the call drop is frequently caused when transmitting a handoff completion message (HCM), mostly due to differences in system design rules.
The conventional procedure of processing the handoff as illustrated in FIG. 1 reduces a call drop rate when the hard handoff is performed, but does not work perfectly, due to differences in the system design rules. In addition, there is a problem in that the conventional procedure can cause unnecessary power consumption and over load by unconditionally transmitting an acknowledgement message when the soft handoff is performed.
Further, a so-called Fast Acknowledgement system, which was first adopted in an IS-2000 system, has introduced an improved system of transmitting an acknowledgement message immediately in an Rx/Tx (Reception/Transmission) (Layer 2) task after a mobile terminal has received a handoff Direction Message (HDM). This Fast Acknowledgement system may efficiently eliminate such call drop phenomena by disabling Tx before transmitting of an Acknowledgement message to a base station in the hard handoff in between frequencies, the Acknowledgement message being sent from a Main Control (MC) (Layer 3) task. However, the Fast Acknowledgement system still has some disadvantages in view of a power saving and overhead, because the Acknowledgement message is unconditionally transmitted irrespective of whether the Acknowledgement message from the Rx/Tx task is soft handoff or hard handoff, and no validation to the message is performed.